1. Field of the Invention
The present invention relates to a power converter for a hybrid power filter. More particularly, the present invention relates to a hybrid power filter for filtering harmonic current which is generated from a single-phase power system, a three-phase, three-wire power system or a three-phase, four-wire power system.
2. Description of the Related Art
Recently, the characteristics of power electronic devices have been improved significantly. The power electronic devices with features of high-voltage rating, high-current rating, and high switching speed have been developed due to the improvement of semiconductor manufacturing technique. Power electronic devices are widely applied in electric power facilities, such as an uninterruptible power supply, motor driver, arc furnace, trolley car, battery charger, and lighting appliance etc. These electric power facilities may generate a large amount of harmonic current due to the nonlinear input characteristic of such loads. The harmonic current will pollute the power system and results in serious problems such as transformer overheat, rotary machine vibration, degrading voltage quality, electric power components destruction, medical equipment malfunction etc.
In order to improve the problems of harmonic pollution effectively, many harmonic control standards, such as IEEE519-1992, IEC1000-3-5, and IEC1000-3-4 etc., have been established by international research centers. In Taiwan, Taiwan Power Company has established a power control provisional standard for harmonic currents. How to solve the harmonic problems is an important topic in today's power system worldwide.
Conventionally, the passive power filter is used to solve the problem of harmonics. The passive power filter generally consists of inductors and AC capacitors. However, the passive power filter causes the problems of resonance and neighboring harmonic current injection that may destroy the passive power filter. Additionally, it is quite hard to obtain a better filtering effect of the passive power filter due to system reactance that may affect the filtering characteristic of the passive power filter.
Many other solutions for the harmonic problem are disclosed in the prior art, such as U.S. Pat. Nos. 5,977,660 and 5,321,598. Referring initially to FIG. 1, which illustrates a schematic circuitry of a conventional active power filter, a power source 1 supplies three-phase electric power to a load 3 with which to connect the active power filter 8 in parallel. The active power filter 8 includes at least one filtering inductor 80, a bridge power converter 81 and a DC capacitor 82. In order to filter the harmonic current generated from the load 3, the active power filter 8 provides a compensation current and injects it into the power line to filter the harmonic current of the load 3. Although the active power filter 8 performs a better filter characteristic, the capacity of the power converter 81 of the active power filter 8 must be greater than the product of the harmonic current of the load 3 and the voltage of the power source 1. Therefore, the capacity and manufacture cost of the power converter 81 of the active power filter 8 must be significantly increased, which limits the wide application of the active power filter 8.
A hybrid power filter has been developed to solve the problems of the passive power filter and the active power filter, as disclosed in U.S. Pat. Nos. 5,567,994 and 5,731,965. Referring to FIG. 2, which illustrates a schematic circuitry of a conventional hybrid power filter. A hybrid power filter consists of a passive power filter 90 and a power converter 91. In operation, the passive power filter 90 is used to lower the capacity of the power converter 91 while the power converter 91 is used to improve the filter characteristic of the passive power filter 90. Advantageously, the power converter 91 can solve problems of resonance and neighboring harmonic current injection caused by the passive power filter 90. Consequently, the hybrid power filter is suitably to a greater capacity of nonlinear load for filtering harmonics.
On account of the disadvantages of the passive power filter and the active power filter, a number of techniques of the hybrid power filters have been applied to a three-phase, three-wire power system described in patents, such as U.S. Pat. Nos. 5,567,994 and 5,731,965. The power converter of the three-phase, three-wire power system employs a three-phase bridge structure and three arms thereof. The three-phase three-arm bridge structure totally employs six power electronic switches and each arm is configured a pair of power electronic switches.
However, there is a need for reducing the number of arms of the three-phase bridge structure to save the manufacture cost. To accomplish this task, a half-bridge power converter is provided. Conventionally, a pair of serially connected capacitors of the half-bridge power converter substitutes for the power electronic switches of one arm of the power converter.
The present invention intends to provide a power converter for a hybrid power filter applied to a single-phase power system, a three-phase, three-wire power system or a three-phase, four-wire system. The power converter has one less arm than the conventional power converter and one less pair of power electronic switches. A power line from the power system directly connects to a DC side of the power converter or through the passive power filter to a DC side of the power converter to reduce the manufacturing cost of the hybrid power filter.